Vertical mills consist of rollers rotating about a vertical axis on a crushing track. The crushing rollers are mounted on a central hub and are provided at their periphery with wear pieces which may take the form of segments or which may also be constituted in monobloc form.
These wear pieces are made of cast iron having a high abrasion strength, especially cast irons having a high chromium content and, consequently, are not ductile at all (their value of elongation at break is virtually equal to zero). The wear pieces are generally fastened with the aid of clamps to the hub. Due to their lack of ductility, cracks or fissures may be observed to appear under the stresses due to the various mechanical stressings, these-cracks or fissures propagating into the said pieces. Under the effect of the stresses, various other drawbacks become apparent, including, in particular, deformation of the hub. This deformation of the hub causes considerable difficulties when replacing the wear pieces and it is common to have to replace both the wear piece and the hub.
In addition, the wear piece is particularly difficult to machine after casting because of its specific properties, namely its high hardness, whereas tolerances lower than H9 are required for these machinings.
In Document GB-2 078 575 A, weldable wear pieces are described which consist of a weldable steel part provided with protrusions which, after having been placed in a mould, undergoes metallurgico-mechanical bonding obtained on casting a cast iron. These pieces are described as being able to be suitable for various uses, such as engineering castings and tools. For the cast wear part, it is proposed to use a spheroidal graphite cast iron of low wear resistance and of low weldability in which a hard metal is incorporated.
Combined metallurgico-mechanical bonding is described as making it possible to compensate for the tensions resulting from the differences in expansion coefficient between the basis alloy (graphitic cast iron) and the reinforcing material incorporated into the wear part, thus ensuring a good bond between this wear part and the weldable part and preventing the propagation of microcracks.
Document AU-B-44415/89 (Acceptance no 617891) describes a method of producing a composite wear resistant product comprising a white iron casting joined to a steel or iron backing, said method including the steps of positioning in a mould at least two steel inserts having protruding locking fingers, casting the white iron over the steel inserts and the protruding locking fingers whereby the inserts are metallurgically and mechanically locked to the white iron, and after cooling of the white iron welding the inserts together to form the backing of the product.
However, the Applicant has noticed that none of the solutions is entirely satisfactory, on the one hand because it limits the use of very hard compositions for the wear part since it is necessary to provide a metallurgico-mechanical bond or welding, and on the other hand because such a bond or welding does not sufficiently prevent cracks and microcracks from propagating but rather facilitates cracks and microcracks. This phenomenon is particularly observed in the case of large vertical mills.